Synthesis and Microwave Absorption Properties of ...

Journal of Science and Technology, Vol. 9 No. 3 (2017) p. 33-40

Synthesis and Microwave Absorption Properties of Doped Expanded Polystyrene with Silver Nanoparticles Adam Rafiq Mohd Arif1, Jibrin Alhaji Yabagi2,3, Mohamad Fahmi Hussin1, Yee See Khee4, Mohammed Isah Kimpa1,5, Muhammad Nmaya Muhammad2,3, Mohd Arif Agam2* 1

Faculty of Electrical Engineering, University Technology MARA Malaysia. Faculty of Applied Sciences and Technology, Universiti Tun Hussein Onn Malaysia. 3* Department of Physics, Ibrahim Badamasi Babangida University Lapai, Niger State Nigeria. 4 Faculty of Electric and Electronic Engineering, Universiti Tun Hussein Onn Malaysia. 5 Department of Physics, Federal University of Technology Minna Niger State Nigeria. 2*

Received 30 September 2017; accepted 4 December 2017; available online 17 December 2017

Abstract: Polystyrene (PS) is found to become a future major environmental issue and aggressive attempt by scientists to recycle PS into useful functional, recycle materials are becoming recent research trend. Polystyrene has been considered as a potential conductive and wave absorber materials. Doping PS with metal nanomaterials are found to enhance PS physical properties, thus enable the fine tuning of its conductive and wave absorbing properties. In this report, expanded polystyrene as source of PS waste materials are dissolved in tetrahydrofuran (THF), later added into deionized (DI) water to form precipitate polystyrene nanoparticles. This procedure enables easy access of dope silver (Ag) nanoparticles into PS in a colloidal form especially for homogeneity consideration. The UV-Vis spectroscopy (Ultra-Violet Visible), Field Emission Scanning Electron Microscopy (FESEM), Fourier Transformation Infrared Spectroscopy (FTIR), and Vector Network Analysis (VNA) are used to investigate the doped PS nanocomposite of its morphology, molecular vibrations, electrical and electromagnetic properties. The results indicate promising effects of the electrical and electromagnetic properties of doped polystyrene metal nanocomposite as potential conductive and wave absorber material. Results show that these properties are highly dependent on the type of doping materials. Keyword: Expanded polystyrene; Tetrahydrofuran; Silver; VNA; FTIR; Microwave absorption

1. Introduction Microwaves are used extensively in wide range applications including radar technology and radio astronomy, wireless local area networks, smart transport and electronic toll collection systems [1], [2]. Microwave absorber in electronic equipment controls the excessive self-emission of electromagnetic waves and also ensures the undisturbed functioning of the equipment in presence of external electromagnetic wave (EM). Microwave absorbers are also highly demanded in defence and aerospace industries, as the application of microwave-absorbing coating on the exterior surfaces of military aircraft and vehicles helps to avoid detection by the radar [3]. Electromagnetic *Corresponding author: [email protected] 2017 UTHM Publisher. All right reserved. penerbit.uthm.edu.my/ojs/index.php/jst

wave absorbers are materials that effectively attenuate the intensity of electromagnetic waves by absorbing them [4]. Polymers have been studied extensively as promising materials for integrated optical devices due to the broad spectrum of possible applications in data transmission and processing systems [5]. They have become a very attractive for photonic devices because they are easily processed, offer high flexibility, tailor-made properties and have a relatively low cost [6]. In conventional linear polymers such as poly methyl methacrylate (PMMA), polystyrene, polycarbonate have showed the great potential for their use as waveguides at relatively short wavelengths [6] . Polystyrene (PS) has been chosen as the polymer matrix due to its good thermal

33


properties, controllable dielectric constant and reasonably light weight [7]. Also PS is widely used in several applications because of its heat durability, nontoxicity, mechanically strength, and thermally non degradable properties. Polymer nanocomposites have attracted great attentions due to the significant roles of nanoparticles (NPs) in enhancing the optical, electrical, magnetic, and mechanical properties [8]. It is well known that the state of particles dispersion is an essentially important in nanocomposites, which is determined by the competition between particle-particle interaction and particle-polymer interaction. Attractive interaction between particles is very common in polymer nanocomposites, which may result in agglomeration of particles and the smaller particle size [7]. Silver nanoparticles (NPs) are used in many applications, such as optics, electronics, gas sensors, catalysis and biomedicine. The properties relevant for these applications depend on the size of the particles [9]. The studies on optical properties of polymers have attracted much attention in view of their application in electronic and optical devices. The optical properties are studied to achieve better reflection, antireflection, interference and polarization properties [4]. There are been recent researches on the microwave dielectric properties and absorption behavior of polymers and composites. In this study, polystyrene and silver nanoparticles were synthesized and their chemical properties were investigated. Also, doped polystyrene with silver nanoparticles derivatives was assessed for application as waveguide absorption. 2. Experimental 2.1

micropipette in a ratio of 1:10, to evaporate the THF the sample was loaded into an oven at 50 oC for 12 hours and later centrifugation was used, to remove water and finally achieved the end product needed. 2.2

Synthesis of Ag nanoparticles

Chemical reduction technique was used to synthesize silver (Ag) nanoparticles using sodium borohydride NaBH4 as a reduction agent. From this technique, 30 mL of 0.002M sodium borohydride (NaBH4) was prepared in an Erlenmeyer flask. Magnetic stir bar was added into the solution and placed into an ice bath on a magnetic stirrer. 10 mL of 0.001M silver nitrate (AgNO3) was added by dropping into stirring NaBH4 solution at approximately 1 drop per second. By mixing both solutions, Ag ions were reduced and clustered to form monodispersed nanoparticles. 0.3 wt% of polyvinyl pyrrolidone (PVP) was added to the solution to prevent agglomeration between the nanoparticles. 2.3

Fabrication of PS/AgNPs

The PS/AgNPs nanocomposite was fabricated through in-situ method, the polystyrene-to-silver nanoparticles ratio varies between 1:4 and 1:8. The samples were then transferred to eppendorf tube for homogeneity and mixed by using a Vortex mixer for one minute at high rpm, later ultrasonic agitation for 45mins at high frequency before coated. Samples were cut precisely to fit the waveguide sample holders in order to minimize the reflection loss via gap between the film and waveguide sample holders. A pure PS film was used as a reference material.

Synthesis of polystyrene particles 2.4

The synthesis of polystyrene particles via nanoprecipitation technique was adopted from Rajeev et al., [10]. Expanded Polystyrene (EPS) waste was obtained from packing material and heated to 150 oC in a hot air oven for 12 hours to remove any volatile matters that may agglutinate. The expanded polystyrene was then weighed and diluted in tetrahydrofuran (THF) at the ratio of 2mg/ml. The EPS/THF was mixed with deionized water using magnetic stirrer and

Characterization techniques

The colloidal Ag nanoparticle was analyzed by UV-Vis measurements with a 1 cm path length quartz cuvette using a UV-1800 Spectrophotometer (Shimadzu) at room temperature in the range 300-900 nm. Morphologies of the samples were observed using Field Emission Scanning Electron Microscopy (FESEM, JEOL JSM-7600F) and coated with platinum before characterization. 34


FTIR analysis was performed using a Perkin Elmer FTIR Spectrometer LR 64912C, N3896 equipped with a universal Attenuated Total Reflectance (ATR) sample stage and a spectrum express FTIR software V1.3.2 Perkin Elmer LX100877-1. The sample molecular structure is determined within the range of 4000–400 cm−1 with a resolution of 4 cm−1. The microwave absorption was carried out using a Vector Network Analyzer (VNA) to determine the values of complex relative permittivity in the 2.0 MHz-13 GHz range by using coaxial reflection technique. During measurement, samples were coated on silicon wafer and mounted in a coaxial sample holder to waveguide adapters which are connected to the ports of VNA. Two ports calibrations were carried out on the adapter surfaces using the standard X band waveguide calibration kit before placing the samples for the measurements.

3. Results and discussion 3.1 UV-Visible nanoparticles

analysis

of

silver

Fig. 1 shows the absorption spectrum of the silver nanoparticles. Sample presents the characteristic surface Plasmon of silver nanoparticles present a narrow band with a maximum at 420 nm. It was reported that the absorption spectrum of spherical silver nanoparticles present a maximum between 420 and 450 nm with a blue or red shift when particle size diminishes or increases [19]–[22]. The results further indicated that the formation of silver nanoparticles via chemical reduction method was very successful.

0.3

Microwave absorption theory

The microwave absorbers are mainly characterized using electromagnetic properties in terms of the complex permittivity and the complex permeability. The complex permittivity arises from the dielectric polarization of the materials and defines as [11]–[15].

     j 

  

0.2

0.1

0.0 300

400

500

600

700

800

900

Wavelength (nm)

(1)

Where  ' is the real part of complex permittivity (known as the dielectric constant) and  " is the imaginary part of complex permittivity [3].The real and imaginary parts of complex permittivity are mainly associated with the stored energy and energy dissipation of the material. Energy loss in a material is depends on the illuminated by electromagnetic waves comes about through damping forces acting on polarized atoms and molecules and through the finite conductivity of a material [16]–[18]. The dielectric loss tangent of the material is defined as

tan  

Absorbance (a.u)

420 nm

2.4.1

(2)

Fig. 1 UV–Vis spectrum nanoparticles synthesized

of

the

silver

3.2 FESEM analysis The morphology of the PS and PS:Ag nanocomposites were studied using the FESEM and results are showed in Fig. 2(a-c), reports have shown that the incorporation of metals nanoparticles into polymer can improve the properties or create a new functionalities [23]. The important factors involved in improving the properties of polymer are the type, size, shape and ratio of the metal nanoparticle dispersion in polymer and interfacial interaction between organic and inorganic nanoparticles, fig. 2a show un-uniform size of polystyrene particles. FESEM image of PS/AgNPs (Fig. 2b&c) clearly reveals that AgNPs are dispersed at the surface and embedded within the polystyrene; the structures 35


show clear boundary within the matrix that silver nanoparticles indicates a weak interfacial interaction between aggregates and PS, which is consistent with the results obtained by FTIR spectrometry. These results are clearly show that the AgNPs were successfully embedded into polystyrene and modified the functional properties of the polystyrene.

(a)

(a)

(b)

(c)

(b)

3.3

FTIR analysis

To confirm the chemical structure of the PS and PS:Ag nanocomposites, FTIR spectra analysis of PS and PS:Ag were performed. Fig 3a-c presents FTIR spectra of the PS, PS:Ag 1:4 and PS:Ag 1:8 nanocomposites. From fig 3a, the spectrum shows the presence of the characteristic bands of PS at 3060 cm−1, 3026 cm−1, which corresponds to the =C-H stretching due to the aromatic ring, bands at 2907 cm−1 2843 cm−1, is attributed to the symmetrical and asymmetrical stretching vibration of CH2, bands at 1600 cm−1, 1494 cm−1, 1445 cm−1 are correspond to stretching vibration of the C=C bond on the benzene ring, bands at 1112 cm−1, 1055 cm−1 and 882 cm−1, which corresponds to the =C-H stretching due to the aromatic ring, and C-H out of plane bending vibration respectively. [24][25]. In addition, the band at 3068 cm−1 is for stretching vibration of O-H, which indicates the existence of hydroxyl. The hydroxyl may appear from water. After doping, Fig. 3(b-c) presents FTIR spectra of the PS:Ag nanocomposites. For the PS/Ag (1:4) nanocomposites, spectrum shows characteristic bands at 1773 cm−1, 1636 cm−1, 1292 cm−1 1084 cm−1 and 877 cm−1 which correspond to the aromatic ring and aliphatic C– H and –CH stretching, respectively. For the PS:Ag (1:8), the spectrum shows characteristic bands at 1773 cm−1, 1636 cm−1 and 1301 cm−1 are assigned to the aliphatic C–H and –CH2 respectively; where some bands were found collapse, reduced in intensity and shifted compared with the PS.

1773

Transmittance (%)

1301

4000

3500

1600 1494 1445

2843 2907

3026

(a)

3000

2500

2000

1500

1112 1055 882

3060

1292

877

1773 1636

1084

(b)

3868

(c) Fig. 2 FESEM images obtained for (a) PS (b) PS:AgNPs (1:4) PS:AgNPs (1:8) nanocomposite.

1636

(c) PS:Ag 1:8 PS:Ag 1:4 PS

1000

Wavenumber (cm-1)

Fig. 3 FTIR spectra of (a) PS (b) 1:4 (c) 1:8 PS:AgNPs nanocomposites. 36


3.4

Microwave absorption

3.4.1

Permittivity spectra analysis

(a)

Permittivity (real part) '

20

15

10 2

4

6

8

10

12

Frequency (GHz) (b)

Permittivity (imaginary part) "

The permittivity of the PS and PS;AgNPs were measured using waveguide method. Usually, the complex permittivity of composite is an equivalent permittivity since it can not only be controlled by conductive filler but also be significantly influenced by polymer matrix. The complex permittivity of composite is decided by volume fraction of conductive filler [11]. However, the volume fraction of conductive filler is influenced by the density of polymer matrix, when weight fraction is used to compare the EM absorption properties. Fig. 4a&b shows the real and imaginary part of the complex relative permittivity spectra for PS and PS/AgNPs (1:4 & 1:8), where the parameter real part  ' represents the charge storage (dielectric constant) whereas the parameter of imaginary part  " is to measure dielectric losses. Dispersion of AgNPs nanoparticles filler in PS matrix leads to interfacial polarization and relaxation effects [26]. All these factors are responsible for high values of real and imaginary part. Fig. 5a for 1:4 and 1:8 shows that as the ratio of AgNPs increases, the dielectric constant increases from 9.03 to 24.28 whereas the imaginary part also increases (fig. 5b) from 0.8 to 8.9. The real part of the complex permittivity is almost constant in the whole X-band of microwave frequency for a given silver nanoparticles concentration, it increases with the significant increase of silver nanoparticles in the polystyrene matrix. The imaginary part of the complex permittivity (loss factor) increases with the addition of silver nanoparticles which could be attributed to the conversion of low loss PS into the lossy dielectric due to presence of high ratio of silver nanoparticles conducting fillers. It can be concluded that the PS/Ag nanocomposite with higher filler content appears to be a promising candidate for microwave absorber applications. The increased in conductivity of PS depends on the presence of Ag nanoparticles which could account for the enhanced reflection of EM wave from both external as well as internal surface.

PS:Ag 1:8 PS:Ag 1:4 PS

25

10 PS:Ag 1:8 PS:Ag 1:4 PS

8 6 4 2 0 2

4

6

8

10

12

Frequency (GHz)

Fig. 4 Frequency dependence of (a) real part (b) imaginary part of relative complex permittivity for PS and PS:AgNPs nanocomposites. 3.4.2

Conductivity and dielectric loss

Polymer matrix has a significant effect on the amount of filler required for electrical percolation [12]-[13],[26]. The electrical conductivity of polymer matrix also influences the permittivity of composite. Fig. 5a&b shows the conductivity and dielectric loss tangent of PS and PS/AgNPs nanocomposite at different ratios. Fig. 5a reveals that for neat PS the conductivity value ranges from (0.08-0.99), for PS/AgNPs 1:4 (1.50-2.97) and for PS/AGNPs 1:8 (3.70-6.90) which indicate that as filler increases the value of conductivity increases. The increase in permittivity can be attributed to the increase in electrical conductivity. Fig. 5b shows dielectric tangent loss factor (tanδ) of PS (0.08-0.19), PS:AgNPs 1:4 (0.34-0.47), and PS:AgNPs 1:8 (0.72-097), in 37


the range of frequency 2-13 GHz. It was observed that in all cases the tanδ value of nanocomposites is higher than neat PS. The higher dielectric loss value for PS:AgNPs is probably due to absorbance of EM waves due to internal reflection. In addition it was also observed that with increasing AgNPs loading, the PS/Ag nanocomposites show increasing dielectric loss. High conductivity of Ag nanoparticles is probably responsible for this observation. Ag has the highest electrical conductivity among other metals so it contains considerable amount of free electrons for movement. As the collision of particles occurs among each other and causes hindrance in movement, during collision energy of EM wave transferred in to heat to be dissipated. This could be responsible for the increments in dielectric loss of the materials [2729]. (a)


Conductivity (s/m)

6

Acknowledgement The author is grateful to Universiti Teknologi MARA (UiTM) and Univerisiti Tun Hussein Onn Malaysia (UTHM) for help with the experiments.

2

References 2

Dielectric loss tangent (tan )

In conclusion, silver nanoparticle was established by chemical reduction method. UVVIS absorption results confirmed formation of silver nanoparticles synthesized by chemical reduction method. PS:AgNPs nanocomposites was successfully prepared by ultrasonic technique. This method provides an alternative approach for the preparation of metal doped polystyrene. FESEM and FTIR results confirm the formation of PS/Ag composites, which had both properties of the polystyrene and surface properties of silver nanoparticles. NVA results revealed that both permittivity and electrical conductivity have showed increases as the filler increases compared with neat PS. PS:Ag nanocomposite with higher filler content appears to be a promising candidate for microwave absorber applications. It is also believed that the prepared PS:Ag composites have other potential applications such as catalysis, antibacterial and optical.

4

0

(b)

4. Conclusion

4

6 8 10 Frequency (GHz)

12

[1]


1.0

[2]

0.8 0.6 0.4 0.2

[3]

0.0 2

4

6

8

10

12

Frequency (GHz)

Fig. 5 Frequency dependence of (a) conductivity (b) dielectric loss tangent for PS and PS:AgNPs nanocomposites with different ratio.

[4]

Qin, F. and Brosseau, C., (2012) “A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles,” J. Appl. Phys., vol. 111, no. 6. Ramesh, G. V Sudheendran, K., Raju, K. Sreedhar, C. B. and Radhakrishna, T. P. (2009) “Microwave absorber based on silver nanoparticle-embedded polymer thin film,” J Nanosci Nanotechnol, vol. 9, no. 1, pp. 261–266. Abbas S. M., Dixit A. K., Chatterjee R., and Goel, T. C. (2007) “Complex permittivity, complex permeability and microwave absorption properties of ferrite-polymer composites,” J. Magn. Magn. Mater., vol. 309, no. 1, pp. 20–24. Beard G. E., Kaynak, A., Unsworth, J. Clout, R. and Mohan, A. S. (1994) “A 38


[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

study of microwave transmission, reflection, absorption, and shielding effectiveness of conducting polypyrrole films,” J. Appl. Polym. Sci., vol. 54, no. 3, pp. 269–278. Salaneck, W. R., Friend, R. H. and Brédas, J. L. (1999) “Electronic structure of conjugated polymers: consequences of electron–lattice coupling,” Phys. Rep., vol. 319, no. 6, pp. 231–251. Becker, M. R., Stefani, V. Correia, R. R. B. Bubeck, C., Jahja, M., and Forte, M. M. C. (2010) “Waveguide optical properties of polystyrene doped with p-nitroaniline derivatives,” Opt. Mater. (Amst)., vol. 32, no. 11, pp. 1526–1531. Suárez, I., Gordillo, H., Abargues, R., Albert, S., and Martínez-Pastor, J. (2011) “Photoluminescence waveguiding in CdSe and CdTe QDs–PMMA nanocomposite films,” Nanotechnology, vol. 22, no. 43, p. 435202. Yu, W., Wang, J., and You, W., (2016) “Structure and linear viscoelasticity of polymer nanocomposites with agglomerated particles,” Polymer (Guildf)., vol. 98, pp. 190–200. Baber, R., Mazzei, L., Thanh, N. T. K. and Gavriilidis, A., (2015) “Synthesis of silver nanoparticles in a microfluidic coaxial flow reactor,” RSC Adv, vol. 5, pp. 95585 – 95591. Rajeev, A., Erapalapati, V., Madhavan, N. and Basavaraj, M. G. (2016) “Conversion of expanded polystyrene waste to nanoparticles via nanoprecipitation,” J. Appl. Polym. Sci., vol. 133, no. 4, pp. 2–6. Watts, P. C. P., Ponnampalam, D. R., Hsu,W. K., Barnes,. A. and Chambers, B. (2003) “The complex permittivity of multi-walled carbon nanotube– polystyrene composite films in X-band,” Chem. Phys. Lett., vol. 378, no. 5–6, pp. 609–614. Watts P. C. P., Hsu, W.-K., Barnes, A., and Chambers, B., (2003) “High Permittivity from Defective Multiwalled Carbon Nanotubes in the X-Band,” Adv. Mater., vol. 15, no. 78, pp. 600–603. Gui, X., Wang, K., Wei, J., Lü, R., Shu, Q., Jia, Y., Wang, C., Zhu, H., and Wu,

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

D., (2009) “Microwave absorbing properties and magnetic properties of different carbon nanotubes,” Sci. China Ser. E Technol. Sci., vol. 52, no. 1, pp. 227–231. Kong, L., Yin, X., Zhang, Y., Yuan, X., Li, Q., Ye, F., Cheng, L., and Zhang, L., (2013) “Electromagnetic Wave Absorption Properties of Reduced Graphene Oxide Modified by Maghemite Colloidal Nanoparticle Clusters,” J. Phys. Chem. C, vol. 117, no. 38, pp. 19701– 19711. Kong, X. L. Yin, X. Yuan, Y. Zhang, X. Liu, L. Cheng, and L. Zhang, (2014) “Electromagnetic wave absorption properties of graphene modified with carbon nanotube/poly(dimethyl siloxane) composites,” Carbon N. Y., vol. 73, pp. 185–193. Lim, K. M., Lee, K. A., Kim, M. C., and Park, C. G., (2005) “Complex permeability and electromagnetic wave absorption properties of amorphous alloy–epoxy composites,” J. Non. Cryst. Solids, vol. 351, no. 1, pp. 75–83. Hotta, M., Hayashi, M., Lanagan, M. T., Agrawal, D. K., and Nagata, K., (2011) “Complex Permittivity of Graphite, Carbon Black and Coal Powders in the Ranges of X-band Frequencies (8.2 to 12.4 GHz) and between 1 and 10 GHz,” ISIJ Int., vol. 51, no. 11, pp. 1766–1772. Nwanya, A. C., Amaechi, C. I., Udounwa, A. E., Osuji, R. U., Maaza, M., and Ezema, F. I. (2015) “Complex impedance and conductivity of agar-based ionconducting polymer electrolytes,” Appl. Phys. A Mater. Sci. Process., vol. 119, no. 1, pp. 387–396. Porel, S., Venkatram, N., Rao, D. N., and Radhakrishnan, T. P., (2007) “In Situ Synthesis of Metal Nanoparticles in Polymer Matrix and Optical Limiting Application,” vol. 7, no. 7. Julkarnain, M., Mondal, A. K., Rahman, M., and Rana, S. (2013) “Preparation and Properties of Chemically Reduced Cu and Ag Nanoparticles,” vol. 2013, pp. 1–3. Dung Dang, T. M., Tuyet Le, T. T., Fribourg-Blanc, E., and Chien Dang, M., 39


[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

(2011) “The influence of solvents and surfactants on the preparation of copper nanoparticles by a chemical reduction method,” Adv. Nat. Sci. Nanosci. Nanotechnol., vol. 2, no. 2, p. 25004. Ramesh, G. V., Porel S., and Radhakrishnan, T. P., (2009) “Polymer thin films embedded with in situ grown metal nanoparticles.,” Chem. Soc. Rev., vol. 38, no. 9, pp. 2646–2656. Gautam, A., Tripathy, P., and Ram, S., (2006) “Microstructure, topology and Xray diffraction in Ag-metal reinforced polymer of polyvinyl alcohol of thin laminates,” J. Mater. Sci., vol. 41, no. 10, pp. 3007–3016. Wibawa, P. J., Saim, H., Agam, M. A., and Nur, H., (2011) “Design, preparation and characterization of polystyrene nanospheres based-porous structure towards UV-vis and infrared light absorption,” Phys. Procedia, vol. 22, pp. 524–531. Yabagi, J. A., Kimpa, M. I., Muhammad, M. N., Uthaman, K. I., Zaidi, E., and Agam, M. A., (2017) “Structural transformation of polystyrene nanosphere produce positive and negative resists by controlled laser exposure,” Adv. Sci. Lett., vol. 23, no. 7. Yuan, C. L. and Hong, Y. S., (2010) “Microwave adsorption of core–shell structure polyaniline/SrFe12O19 composites,” J. Mater. Sci., vol. 45, no. 13, pp. 3470–3476. Arjmand, M., Mahmoodi, M., Gelves, G. A., Park, S., and Sundararaj, U., (2011) “Electrical and electromagnetic interference shielding properties of flowinduced oriented carbon nanotubes in polycarbonate,” Carbon N. Y., vol. 49, no. 11, pp. 3430–3440. Zhao, H., Sun, X., Mao, C., and Du, J., (2009) “Preparation and microwave– absorbing properties of NiFe2O4polystyrene composites,” Phys. B Condens. Matter, vol. 404, no. 1, pp. 69– 72. Saini, P., Choudhary, V., Singh, B. P., Mathur, R. B., and Dhawan, S. K., (2011) “Enhanced microwave absorption

behavior of polyaniline-CNT/polystyrene blend in 12.4–18.0GHz range,” Synth. Met., vol. 161, no. 15–16, pp. 1522–1526.

40